Helicopters are equipped with many sensors that advise the pilot of the condition of various on board systems. There are normally open sensors, which close the circuit when the sensor is activated, and normally closed sensors, which open the circuit when activated. The sensors are connected to indicators such as warning lights on the pilots instrument panel. The sensor is typically located remote from the indicator. Therefore, the electrical connections typically pass through several harnesses, junction boxes, terminal boards, etc.
An example of a normally open sensor is a chip detector. A chip detector is used to monitor the health or air worthiness of a helicopter's transmission or gear box, which is a vital piece of equipment. The presence of a significant number of metal chips in the transmission fluid usually indicates mechanical problems with the transmission. The chip detector is partially immersed in the transmission fluid so as to be exposed to the metal chips circulating inside of the fluid. The chip detector is provided with a magnet so as to attract and retain the metal chips. The presence or absence of metal chips captured by the chip detector is indicated both visually and electrically. The electrical indication is provided by a warning light on the instrument panel. If metal chips accumulate during flight, the warning light is illuminated and the pilot can safely land the helicopter, before the rotors lock up.
The visual indication is provided between flights by a ground mechanic. The mechanic physically removes the chip detector from the transmission, visually inspects the collection area on the chip detector for metal chips, and then reinstalls the chip detector into the transmission. A visual inspection of the chip detector is required after the helicopter is flown for a specified number of hours. (In Canada, the chip detector is required to be visually inspected every day.)
Several problems can and have arisen due to the frequent removal and installation of chip detectors. Because the chip detector is in contact with transmission fluid, failure to properly reinstall the chip detector could result in a loss of fluid during flight. In fact, this very problem occurred in a helicopter flying over the Gulf of Mexico. The loss of transmission fluid during flight resulted in a forced landing of the helicopter on the water. One of the flotation devices on the helicopter failed, resulting in the helicopter flipping over and sinking.
Thus, with the required frequent handling of the chip detector component of the transmission, the possibility for loss of life or aircraft due to human error is significant. The electrical indication circuit provides no clue as to improper installation of an open circuit sensor such as a chip detector. What is needed is a system for detecting the improper installation of a chip detector.
Another problem caused by frequent handling of the chip detector is broken wires. Wires lead from the chip detector to the warning light in the cockpit instrument panel. These wires can be easily broken as the chip detector is handled during the visual inspection process. A broken wire results in the disablement of the electric circuit. In the prior art, there is Berrier, et al., U.S. Pat. No. 5,045,840, owned by the assignee of the present invention. Berrier, et al. provides a continuity sensor that can be installed across an open circuit device such as a chip detector. Upon the application of power to the circuit in the cockpit, the continuity sensor temporarily closes the circuit to illuminate the warning light. If the warning light illuminates, the interconnections leading from the warning light to the chip detector are in working order. However, if the warning light fails to illuminate, then the chip detector circuit is inoperable.
The Berrier, et al. continuity sensor has proven to be a noteworthy and much needed device. Before the Berrier, et al. continuity sensor, prior art electrical sensing circuits with normally open sensors were vulnerable to open circuit faults. With the Berrier continuity sensor, such open circuit faults can be identified and corrected.
It is desired to supplement the Berrier, et al. continuity sensor to, as mentioned above, detect if a chip detector has been improperly installed. In addition, it is desired to provide a system to monitor the continuity of wires leading to the chip detector on a continuous basis. Further still, it is desired to provide a system to monitor the electrical status of the chip detector or other open circuit sensor so as to detect degradation of the contacts.